In a cryogenic air separation unit (ASU), a backup system may be necessary to supply a gaseous product, for example oxygen, nitrogen or argon, in case of shutdown of the ASU for a limited period. The backup system is generally composed of a cryogenic liquid storage and a liquid vaporization system.
Different vaporization systems can be used depending on the atmospheric conditions, on the production capacity, etc. (e.g. an exchanger with a heat transfer fluid such as steam or water, a vaporizer of pool type heated by steam, an atmospheric vaporizer, etc.).
An atmospheric vaporization system constitutes an economical option and offers the advantage of using the “free” heat while it is in service. However, its main drawback is that it has a limited service time because of the icing on the outer surface of the vaporizer because of the presence of moisture in the air.
The de-icing of an atmospheric vaporizer can be done naturally if the temperature of the air is hot enough, for example above 0° C. However, if the ambient temperature may remain cold for a long period (for example a few weeks or even a few months), the installation of a heating system for the de-icing of the vaporizer may prove necessary.
The conventional de-icing methods consist in installing either a dedicated reheater (generally electrical) in line or an electrical reheater outside of the vaporizer but within a closed enclosure, combined with forced ventilation. These de-icing means present a significant cost overhead and, in the case of an in-line electrical reheater for an oxygen vaporizer, there is also a potential risk of high temperature oxygen fire.